Pressure control is an important component of a wide range of industrial processes. One type of process where pressure control is critical is semiconductor dry etch processes. Dry etch processes are used to etch patterns in semiconductor wafers. In a dry etch process, etch gases are fed into a chamber containing semiconductor wafers. The amount of the etch gases in the chamber must be carefully controlled to provide the desired etch characteristics. After the pressure in the chamber reaches a predetermined level, radio-frequency radiation ignites the etchant gases to create a plasma. The pressure in the chamber directly affects the etch process. A pressure above the desired level can cause the process to over etch the wafers. Conversely, a pressure below the desired level can cause the process to under etch the wafers. It is also important that the pressure be maintained at a constant level during the etch process. Pressure variations during the etch process can cause defective etching.
In one conventional approach to controlling pressure in a chamber, the chamber is equipped with a pressure sensor, a gas inflow controller, and a controllable throttle valve. The throttle valve separates the chamber from a vacuum pump. In a typical manual operation the inflow controller is set to a desired flow value, for example 100 sccm. The vacuum pump is then activated, and the throttle valve is adjusted until a desired pressure level, such as 10 mTorr is reached. In an automated approach, if the chamber pressure is higher than desired, an automatic controller gradually opens an exhaust valve to release gas from the chamber until the desired pressure is reached. This approach is unsuitable for many applications because it is generally limited to specific operating conditions outside of which its performance deteriorates rapidly. For example, changes in the desired pressure level, the gas flow level, or the type of gas to be used can render the performance of such systems unacceptable for high performance industrial systems such as semiconductor manufacturing systems. Furthermore, this process is slow and difficult to automate because the optimum valve position for a particular process recipe is dependent on a large number of factors.
An alternative approach to controlling pressure is to use a "calibration" based method. In calibration methods the performance of the equipment is characterized at each potential operating point over each of the operating parameter ranges. Such operating parameters may comprise the type of gas in the chamber, the rate of gas flow into the chamber, and the desired pressure. The valve position and the chamber pressure is recorded at each of these operating points. Generally the relationship between the valve position and the chamber pressure is a highly non-linear relationship. This non-linear relationship is a primary factor that renders linear controllers inadequate to efficiently control pressure.
The calibration process is used to determine the value of each of the operating parameters required to achieve the desired operating conditions. In a manufacturing process, the desired operating parameter values are retrieved for a given operating point and the valve is moved to the appropriate position to produce the desired chamber pressure. If all of the operating parameters are at the expected values then the pressure should reach the desired level based on the valve position setting. Minor errors between the actual and desired pressure can then be corrected using a slow automatic controller.
One problem with such calibration based approaches is that each time a process recipe is modified the operator must go through a lengthy and tedious calibration process. The calibration process wastes valuable production time. The calibration process also uses large numbers of wafers and large quantities of gases to evaluate the system at the desired operating points. Furthermore, such calibration based methods are highly vulnerable to failure when there are variations in the process conditions that are unknown to the operator.
FIG. 2 illustrates the pressure response over a range of different pressure setpoints of a conventional pressure control system using a proportional-integral controller. The FIG. 2 plot of pressure vs time illustrates the inconsistent behavior of a conventional proportional-integral controller system. In the FIG. 2 plot the pressure setpoint is changed at 20 second intervals. The system response at low pressures is slow. For example, the system requires over 20 seconds to adjust the pressure from 0.5 mTorr to 1 mTorr. At higher pressures the system response overshoots the new pressure setpoint and rings badly thereafter. In a semiconductor manufacturing process, this overshoot and ringing is manifest as variations in the pressure of the chamber containing the wafer under fabrication. During a wafer etch process such pressure variations cause undesirable variations in the etch process. Consequently to avoid defects, dry etch processes typically wait until the pressure stabilizes before igniting the gas to create a plasma. This delay decreases the throughput of the manufacturing equipment and thereby increases the cost of the manufactured devices.
Thus an improved method and apparatus for controlling pressure that overcomes these and other problems of the prior art would be highly desirable.